U.S. Pat. Nos. 4,485,297 issued Nov. 27, 1984, 4,523,085 issued June 11, 1985, and 4,542,285 issued Sept. 17, 1985, and U.S. Pat. applications Ser. Nos. 478,080 filed Mar. 23, 1983, now abandoned, and 796,012 filed Nov. 7, 1985, all of which are incorporated herein by reference, disclose electrical heaters of the type including a paper or plastic substrate on which is printed a semiconductor pattern (typically a colloidal graphite ink) having (a) a pair of conductor contact portions extending parallel to each other and (b) a heating portion (typically a plurality of transverse bars) extending between and electrically connected to the conductor contact portions. A metallic conductor (typically copper stripping) overlies each of the conductor contact portions, and an overlying sealing layer is bonded to the substrate closely adjacent the opposite edges of the conductor and holds the conductor in tight face-to-face engagement therewith with the underlying conductor contact pistons.
Typical uses of such heaters include area (e.g., ceiling or floor) heaters, pizza box heaters, thin heaters for pipes, wide heaters for under desks and tables, spaced heaters for greenhouse plant use, and military thermal signature targets.
There are, however, some applications in which the heater design disclosed in the aforementioned patents and patent applications is not entirely satisfactory. For example, in heaters in which the heating area is very small, it is difficult to confine heating to the heated area and there may be too little semi-conductor free area to insure secure tie-down of the metal conductors. Using the copper strip structure of the above-mentioned patents, it is similarly difficult to provide an extremely flexible heater, as is desired for use in, for example, an electric blanket; and the structure of those patents also effectively limits the locations at which electrical contacts may be connected to the heater.